Wi-Fi®, which is a registered trademark of the Wi-Fi Alliance®, is using ever wider bandwidths. Wi-Fi® started out with a bandwidth of 20 megahertz (MHz), moved to 40 MHz, and current standards are approaching 80 MHz. However, the Wi-Fi® bandwidth is in an “all or nothing” fashion. In other words, a Wi-Fi®-enabled device typically either uses the entire spectrum that is allocated to the device or uses none of it.
There are three main challenges with using ever-wider bandwidth as a contiguous Wi-Fi® channel. A first challenge is that the efficiency of the Wi-Fi® system goes down. Even though speeds may go up as the bandwidth increases, the actual end user throughput is actually significantly lower. The reason for this is that existing Wi-Fi® devices are mandated to reduce their channel width immediately upon detecting any coexisting Wi-Fi® device. Thus, Wi-Fi® devices are often not able to efficiently use their full bandwidth capabilities. Moreover, the Wi-Fi® device has to pause and listen for other devices before transmitting. This means that the pause time starts dominating.
A second challenge is that wide channels tend to create unfairness. This is because wide channels tend are more prone than narrow channels to overlap with neighboring networks. For example, neighboring houses can have overlapping networks. There can be unfairness between networks, such that some networks may get little or no throughput. Thus, when coexisting Wi-Fi® networks operate over different but overlapping Wi-Fi® channels, significant unfairness, including starvation, is possible.
A third challenge occurs when there is only a non-contiguous frequency spectrum available. In this situation, there may not be the full bandwidth available in the white space. White spaces are frequencies allocated to a broadcasting service but not used. By way of example, there may be 40 MHz of white space available but in the middle there may be 6 MHz blocked out (such as being used by a transmitting television station). In this case, current Wi-Fi® devices cannot use the 40 MHz spectrum efficiently or even operate there.
Some existing techniques do make use of narrow channels. However, these techniques lack independence. In particular, some techniques transmit on all channels simultaneously. Other techniques either transmit all at the same time or receive all at the same time. Still other techniques tie the channels together so that the channels are not independent. This tends to decrease the efficiency of the Wi-Fi® device.
Efficiency is decreased when one narrow channel is dependent upon another narrow channel. This is because using interdependent narrow channel techniques do not allow fairness between channels and do not allow interoperation when there are multiple access points. Thus, while these techniques make use of narrow channels, the interdependence between the channels can mitigate some of the benefits of using narrow channels.
Some existing techniques require the channels to be synchronized. One such technique uses fine-grained subchannels that allow each node to use only a part of the frequency spectrum. However, the use of subchannels requires a synchronous system, meaning that all nodes transmit within a few microseconds of each other. This makes it difficult for this technique to coexist with unsynchronized neighboring networks.